"The family of the _Neritacea_," said Mr. Elliot, as he renewed his lessons to Charles and Lucy, "are remarkable in their form. Their left-margin is truncated, without any appearance of a columella. They possess an operculum, and are either marine or fluviatic. The genera are, _Navicella._ _Neritìna_ _Nerìta_, Nerite, or hoof-shell. _Nerìta_ _Nerìta_. _Natìca_ _Nerìta_. "You will recollect that the order _Trachelipoda_ is still continued. "_Navicella_ is a foreign genus from the Indian rivers. [Sidenote: NERITINA. NERITA.] "_Neritìna_ is a pretty genus of shells, from the European, the East and West Indian rivers. They resemble the _Nerìta_ genus, but are all fresh-water shells; thin, smooth, and variously marked; without any tooth or notch on the right-margin of the aperture. "_N. virginea_ is common in collections; it is marked with various lines and dots. "_N. fluviatilis_ is common in our rivers: you may find plenty in river-sand, of red and brown colours, and various sizes. "_N. zebra_ and _N. meleagris_ are also pretty shells. The little _Neritìna viridis_, from the West Indian streams, is one of the smallest species, of a pale pellucid green. "_Nerìta_ is a marine genus. The shells are solid and semi-globose; the left-margin is truncated, the right-margin toothed, or crenulated. This genus is never umbilicated. "_N. polìta_ is a handsome shell: it is thick, polished, and variously marked; the base of the aperture is yellowish. "_N. peloronta_, the bleeding-Nerite, is marked with a crimson spot. "_N. tessellata_ is sulcated, or furrowed, chequered with black and white. "_Natìca_ differs from the former genera in these particulars: the shell is umbilicated; the left-margin oblique, not toothed, callous, the callosity sometimes covering the umbilicus. The species are numerous, and several are common in collections. "_N. aurantius_ and _N. millepunctata_ are good shells. [Sidenote: IANTHINA. SIGARETUS. STOMATELLA.] "_Ianthina_ is the last of the snail-like genera. Its beautiful purple colour renders the shell a favourite. They are marine, though so fragile and transparent. The animal floats upon the surface of the sea, by means of a vesicular appendage to the foot, which, it is said, may be inflated or contracted at pleasure. _Ianthina_ shines by night. _I. communis_ is found in abundance in the Atlantic and in the Mediterranean. * * * * * "The family _Macrostoma_ contains, _Sigaretus_ _Helix_? _Stomatella._ _Stomatia._ _Haliòtis_ _Haliòtis_, sea-ear. "These genera form a beautiful family, and all bear a resemblance to the human ear. "_Sigaretus_ is white and pearly; the shell is enveloped in the folds of the mantle belonging to the animal. There are several species, one or two of which were ranked among the _Helix_ race.